The coils or inductors in induction heating are required to produce alternating magnetic fields of very large intensities (in the range 80,000 to 300,000 amperes turns per metre). In the present state of the art almost all induction heating coils are made of hollow copper conductors, which are wound into a single layer solenoidal coil. Because the coil consists of only a single layer of rather large conductor, the number of turns must be small and therefore the current in each turn must be very high to achieve the field intensities required. This gives rise to very large I.sup.2 R losses in the reactor and therefore the efficiency with which energy is transferred from the coil to the billet being heated is low (typically in the range of 30 to 70 percent depending upon the material being heated and the frequency being used). The addition of a second layer of hollow conductors forming a second solenoid concentric with the first and connected in series with it, allows the current in the coil to be reduced to nearly half of its normal value and still maintain the same field intensity at the billet inside the coil. This has the effect of reducing the I.sup.2 R losses in the coil but, unfortunately, the inner layer of hollow copper conductors is heated by the induced currents caused by the field of the outer layer and the resulting losses in the coil are substantially the same as though a single layer coil were used. The addition of even more layers can in fact make the resulting total coil loss larger than it would be for the single layer coil which produces the same magnetic field intensity.
It has long been the goal of induction heating designers to increase the efficiency of their installations and a specific goal has been to devise a method of using multiple layers in a coil to achieve this end. One solution has been described by I. A. Harvey in a paper entitled "a method of improving the energy transfer in induction heating process and its application in a 1 MW billet heater", published in 1977 in IEE Conference Publication 149: Electricity for Materials Processing and Conservation pp. 16-20. The method utilizes a disc wound transformer type coil made from strip type conductors arranged so that the strips are thin in the radial direction and long in the axial direction of the coil and the whole assembly is immersed in water for cooling. This has the effect of reducing the eddy losses near the mid-plane of the coil, where the flux is axial and faces the thin side of the strips but it does not reduce the losses near the end of the coils where a significant portion of the magnetic field is radial. Coils of this construction perform reasonably well at low frequencies but perform very poorly at moderate and high frequencies where the eddy losses are still very substantial. A further disadvantage is the necessity to place all of the conductors in series giving rise to a very high coil voltage. This is particularly troublesome since the insulated coil is immersed in water.
Another proposal was presented in a paper presented at the Electroheat Congress in Stockholm in June 1980 entitled "Technical Innovation in the Induction Reheating of Billets Wires and Strips", by M. Coevet, J. Heurten, J. Nun and E. Poirout, which discloses an induction heating coil wound using a rectangular conductor which comprises 18 transposed insulated subconductors, 12 of which are thin strips and 6 of which are hollow rectangular copper conductors, the latter being interleaved with the former to cool the conductor. The authors claim an improvement in efficiency when heating aluminum at 50 Hz of 12% (from 42 to 54%) and point out that the use of this special conductor is limited to 400 Hz.